(a) The Invention
This invention relates to a heat-recoverable composite tubular article. More particularly, the invention relates to a heat-recoverable composite tubular article formed of an inner layer of an elastomeric material and an outer layer of a thermoplastic material, and to a method of manufacturing the article by coextrusion.
(b) Background
Polymeric heat-recoverable articles are known and can be used for insulating and protecting electric components, such as cables and especially for enclosing joints between two electric power cables and cable terminations.
Such articles may comprise more than one layer of polymeric material. In U.S. Pat. No. 4,373,554 to Cook a heat-recoverable article having one lamina which is relatively fusible and a second lamina which is relatively infusible is prepared by compound extrusion. There is an intimate bond between the lamina. Each lamina is of a thermoplastic polymeric material. U.S. Pat. No. 3,669,824 to Hess describes a similar composite tubular article which is then helically slit to form a tape.
In U.S. Pat. No. 4,383,131 to Clabburn an enclosure or protective sleeve for a shielded cable termination or joint is described which enclosure comprises a conductive outer layer, an insulating inner layer and, optionally, an innermost stress grading layer. It is disclosed that one or more of the layers of the enclosure may be elastomeric. It is also disclosed that the enclosure may be made by coextrusion. However, the only specific examples of a coextruded product contain only thermoplastic layers.
In U.S. Pat. No. 4,390,745 to Bottcher et al. an enclosure for electrical apparatus is described comprising a first hollow sleeve of insulating material around which is positioned a second sleeve comprising an inner layer of insulating material and an outer layer of conductive material. The outer layer of the second sleeve may be thermoplastic and the inner layer, may be elastomeric. Preparation of such an article by coextrusion is not specifically taught.
In Great Britain Pat. No. 1,434,719, to Penneck, and U.S. Pat. No. 4,035,534 to Nyberg, herein incorporated by reference, there is disclosed an article comprising a laminate of an elastomeric layer which is deformed from its equilibrium configuration and a non-elastomeric thermoplastic layer which retains the elastomeric portion in the deformed configuration until heat is applied. The article is prepared by molding and bonding the layers together with heat and pressure. It is taught that the bond between the layers is improved with the use of adhesives. It has been found that in the manufacture of such articles by this method, an adhesive layer is usually required to maintain the bond between the layers during deformation (e.g. expansion) of the article. Further, where the product may be stored prior to use for a period of time, particularly at temperatures above about 50.degree. C., the adhesive prevents separation of the layers of the article as the deformed elastomer layer attempts to prematurely recover.
In summary, Nyberg and Penneck disclose heat-recoverable composite articles comprising an elastomer layer and a thermoplastic layer. The method disclosed for manufacturing such articles comprises a molding technique and generally an adhesive is required to bond the layers together. Clabburn and Bottcher et al. disclose heat-recoverable articles for use in enclosing high voltage (i.e. above 1 kV) joints and terminations. The heat-recoverable articles used can be composite articles containing elastomeric and thermoplastic layers as in Nyberg and Penneck. Cook, Clabburn and Bottcher et al. discuss the preparation of composite heat-recoverable articles by compound extrusion or coextrusion, but refer to the use of thermoplastic materials in this context.